1. Field of the Invention
The present invention relates to a resonator for suppressing intake noises.
2. Description of the Related Art
A Helmholtz type resonator is provided with a cylindrical member and a housing. The cylindrical member is branched and connected at its one end to an intake duct. This intake duct is defined into an intake passage. This intake passage is opened at its cylindrical member connecting portion to form an opening. The housing is connected to the other end of the cylindrical member. The inside of the housing is defined into a volume portion.
Here, the resonance frequency f of the resonator can be obtained for a sound velocity C, an opening area S of the opening, a length L of the cylindrical member and a volume V of the volume portion from the following Formula:
                    f        =                              C                          2              ⁢              π                                ⁢                                    S                              V                ·                L                                                                        [                  Formula          ⁢                                          ⁢          1                ]            
Here, the frequency of the intake noises varies in proportion to the engine speed. In JP-A-2001-50127, therefore, there is introduced a Helmholtz type resonator, which can vary the opening area S of an opening in accordance with the engine speed. From the aforementioned Formula, the resonance frequency f of the resonator can be varied if the opening area S is varied. In the case of the resonator described in that patent publication, at every engine speeds, the sound-pressure level near the frequency F is lowered by equalizing the resonance frequency f to a desired frequency F of the intake noises.
The intake noises are composed of a plurality of components corresponding to the explosions of the combustion chambers of the engine. FIG. 17A plots the primary explosion components of the intake noises of a four-cylinder engine, and FIG. 17B plots the secondary explosion components of the intake noises of the four-cylinder engine. In case the engine speed is 5,000 rpm, in the primary explosion components of FIG. 17A, the frequency to be reduced in the sound-pressure level is 250 Hz. In the case of the same engine speed, in the secondary explosion components of FIG. 17B, the frequency to be reduced in the sound-pressure level is 500 Hz.
In the case of the resonator described in the publication, however, at an arbitrary single engine speed, only one frequency range can reduce the sound-pressure level. When the intake noises at the engine speed of 5,000 rpm are to be reduced by using the resonator of that publication, therefore, it is possible to reduce the sound-pressure level in the neighborhood of the frequency of 250 Hz (of the primary explosion components) but not the sound-pressure level in the neighborhood of the frequency of 500 Hz (of the secondary explosion components). Alternatively, the sound-pressure level in the neighborhood of the frequency of 500 Hz (of the secondary explosion components) can be reduced but the sound-pressure level in the neighborhood of the frequency of 250 Hz (of the primary explosion components) cannot. In case the sound-pressure levels of the two frequency ranges are to be reduced, therefore, it is necessary to arrange a plurality of resonators in the intake duct.
In order to solve this problem, JP-A-5-18224 introduced the resonator which can reduce the sound-pressure levels in two frequency ranges at an arbitrary single engine speed. The inside of the housing of the resonator disclosed is partitioned by a movable partition into a first volume portion and a second volume portion. The first volume portion is connected to the intake duct through a first cylindrical member. The second volume portion is connected to the intake duct through another second cylindrical member. The volume V1 of the first volume portion and the volume V2 of the second volume portion can be changed by movable the movable partition. According to the resonator disclosed, therefore, it is possible to separately set a resonance frequency f1 relating to the first volume portion and a resonance frequency f2 relating to the second volume portion.
However, the volume V1 of the first volume portion and the volume V2 of the second volume portion cannot be varied independently of each other. If the total volume is designated by Vt, a relation of Vt=V1+V2 holds. As a result, the volume V2 necessarily becomes smaller if the volume V1 is increased. If the volume V2 is increased, on the contrary, the volume V1 necessarily becomes smaller.
If this event is substituted for the Formula 1, the volume V1 has to be reduced in case the resonance frequency f1 relating to the first volume portion is shifted to a higher frequency side. If the volume V1 is reduced, however, the volume V2 inevitably increases. Therefore, the resonance frequency f2 relating to the second volume portion inevitably shifts to the lower frequency side. In short, the resonance frequency f1 and the resonance frequency f2 shift in the opposite directions.
On the contrary, the volume V1 has to be enlarged in case the resonance frequency f1 relating to the first volume portion is shifted to a lower frequency side. If the volume V1 is enlarged, however, the volume V2 inevitably decreases. Therefore, the resonance frequency f2 relating to the second volume portion inevitably shifts to the higher frequency side. In short, the resonance frequency f1 and the resonance frequency f2 shift in the opposite directions.
Thus, according to the resonator described in the same publication, the two resonance frequencies f1 and f2 shift in the opposite directions. However, the frequency of the intake noises vary in proportion to the engine speed, as described hereinbefore. In case the engine speed is 2,000 rpm in FIGS. 17A and 17B, for example, the frequency of the primary explosion components of FIG. 17A is 100 Hz, and the frequency of the secondary explosion components of FIG. 17B is 200 Hz. In case the engine speed is 4,000 rpm, on the other hand, the frequency of the primary explosion components of FIG. 17A is 200 Hz, and the frequency of the secondary explosion components of FIG. 17B is 400 Hz. According to the resonator of the same publication, therefore, it is difficult at an arbitrary engine speed to make the two resonance frequencies f1 and f2 correspond to frequencies F1 and F2, the sound-pressure levels of which are to be lowered.
On the other hand, JP-A-2002-21659 introduces a dual intake system for retaining two intake passages by two intake ducts. According to this intake system, it is possible to feed the combustion chambers of the engine with much intake air.
If the dual intake system is adopted, however, the more parts such as an intake duct, an air cleaner or an air cleaner hose are required for the intake passages. Therefore, the assembling works become complicated. The more parts number makes their space the larger. As a result, the limited space in the engine room is narrowed by the dual intake system. Moreover, the large parts number complicates the structure.
As the engine speed grows the higher, on the other hand, the combustion chambers of the engine demand the more intake air. In case the engine speed is low, therefore, the intake passage of only one line can feed the combustion chambers with the intake air of a desired amount. Of the two intake passages of the dual intake system, therefore, one intake passage is excessive, only in case the engine speed is low.